Field of the Invention
The present invention relates to the field of fetal genetic screening and to the field of quantitative nucleic acid analysis.
Related Art
It is now recognized that fetal DNA sheds from the placenta and mixes with the mother's blood at fairly high levels—between 3% and 6% of DNA in the mother's blood is from the fetus. This observation has been used in conjunction with PCR assays for a variety of fetal genetic screens—gender, Rh, and thalassemia. However, the technique remains limited for two primary reasons: first, the PCR assays trade off sensitivity for specificity, making it difficult to identify particular mutations, and second, the most common genetic disorder, Down Syndrome, is a chromosomal trisomy and therefore cannot be detected by conventional PCR in a mixed sample.
It has now been found that these problems can be solved by quantitative examination of large numbers of chromosome samples through the use of highly scalable techniques. This approach is termed here “digital analysis,” and involves the separation of the extracted genomic material into discrete units so that the detection of a target sequence (e.g., chromosome 21) may be simply quantified as binary (0, 1) or simple multiples, 2, 3, etc. The primary example of a technique that can be used to yield such “digital” results is “digital PCR,” which allows efficient amplification from single molecules, followed by subsequent quantitative analysis. Digital PCR, as the term is used here, refers to a quantitative, limited dilution of a nucleic acid sample, such as into multiwell plates, then the amplification of a nucleic acid molecule in a well, which due to the dilution, should be either 0 or 1 molecule. Digital PCR using multiwell plates has been used previously to detect rare mutations by either serial analysis of single molecule (i.e., clonal) amplicons (Vogelstein B, Kinzler K W. Proc Natl Acad Sci USA. 1999 Aug. 3; 96 (16): 9236-41) or by enhancing the sensitivity of differential amplification (www (dot) fluidigm.com/didIFC.htm). Described below is an invention whereby digital PCR can be applied to noninvasive fetal diagnostics in order to detect fetal mutations with specificity and sensitivity beyond what is possible with conventional PCR analysis.
Furthermore, as also described in connection with the invention described below, digital PCR can be used to detect aneuploidy, such as the trisomy that causes Down Syndrome. Since aneuploidies do not present a mutational change in sequence, and are merely a change in the number of chromosomes, it has not been possible to detect them in a fetus without resorting to invasive techniques such as amniocentesis or chorionic villi sampling (Science 309, 2 Sep. 2005 pp. 1476-8).
Another form of digital PCR has been described as emulsion PCR, which has been used to prepare small beads with clonally amplified DNA—in essence, each bead contains one amplicon of digital PCR. (Dressman et al, Proc Natl Acad Sci USA. 100, 8817 (Jul. 22, 2003)).
Another form of Digital PCR can be carried out using microfluidics. In this embodiment, described below, DNA is diluted and separated into small, discrete samples for forming reaction samples by a series of channels and valves.
An example of a suitable method for single molecule analysis that may be adapted to the present methods is given in Braslaysky et al., “Sequence information can be obtained from single DNA molecules, Proc. Nat. Acad. Sci. 100(7): 3960-3964 (2003), which uses sequential incorporation of labeled nucleotides onto an immobilized single stranded DNA template and monitoring by fluorescent microscopy.
Another aspect of the relevant art involves sample preparation in order to carry out the present processes. That is, the fetal DNA may be enriched relative to maternal DNA. Chan, et al., “Size Distribution of Maternal and Fetal DNA in Maternal Plasma,” Clin. Chem. 50(1): 88-92 (2004) reports that plasma DNA molecules are mainly short DNA fragments. The DNA fragments in the plasma of pregnant women are significantly longer than DNA fragments from non-pregnant women, and longer than fetal DNA.